Current responsive instrument



CURRENT RESPONSIVE INSTRUMENT Filed April 28, 1939 Il Q@ MAGNETIC CIRCUIT WITH ExcEssIvE cU NT.

TENDs To sHUNT THE 2 AIR GAP WITH I OW CURRENT 1N THE com |3, BUT BEooMEs SAT- RAoTwlTl-I IEIGCI-(ID UR N |N TH n 1,.

Inventor: Ralph IVI. Rowe I I,

I-I'IIS Attorney.

Patented Aug. 6, 1940 CURRENT RESPONSIVE INSTRUMENT Ralph M. Rowell, Lynn, Maso., 'assignor to General Electric Company, a corporation of New York Application April 28, 1939, Serial No. 270,579 3 0mm. (ci. 1v1-o5) My invention relates to current responsive instruments and concerns particularly instruments and current responsive devices of the dynamometer type.

5 It is an object of my invention to provide an improved iron core type of dynamometer instrument having a high current range and substantial freedom from voltage and current errors and in which the movable coil remains at a zero posil tion and does not produce any torque when no` current is flowing in the field coil, thus obviating the necessity for auxiliary or torque compensating coils.

Other and further objects and advantages will l become apparent as the description proceeds.

In carrying out my invention in its preferred form I provide a field structure having an outer pole piece and an inner pole piece, which is very nearly surrounded by the outer pole piece and spaced to form an annular air gap 250 in length through which an eccentrically mounted coil may rotate, 'Ihe inner piece has an opening through the center, through which pass one side of the coil and the shaft supporting the coil. 'Ihe core member including the inner pole piece is divided so as to have the general form of a U. There are arms or branches, which extend from the outer pole piece to the ends of the U to complete the magnetic circuit of the field. However, air gaps are interposed between the inner pole piece and the arms leading from the outer pole piece in order to increase the reluctance in the path of magnetic flux which tends to be produced by the movable coil. A stationary or iield coil is wound around the neck or base portion of the outer pole piece. In the case of a wattmeter, for example, the field coil may be formed as a current coil and the movable coil as a potential coil of the instrument. The invention may be understood more readily from the following detailed description when considered in connection with the accompanying drawing and those features of the invention which are believed to be novel and patentable will 4,5 be pointed out in the claims appended hereto. In the drawing, Fig. 1 is a front elevation, partially in section, with certain portions of the apparatus broken away for the sake of clarity, showing the laminated-core torque-producing unit of a dynamometer type instrument constructed in accordance with my invention. Fig. 2 is a side elevation of the arrangement of Fig. 1, also shown partially in section, with a portion of the apparatus broken away. Figs. 3 and 4 are fragmentary detail views 'of two different shapes of laminations used in the arrangement of Figs. 1 and 2; and Fig. 5 is 'a graph explaining the principle of operation of my apparatus. Like reference characters are utilized through the drawing to designate like pltS. 6

Referring more in detail to Figs. 1 and 2, in the arrangement described. there is a movable coil II arranged to travel in an annular gap I2 which is of considerable length constituting the greater part of the periphery of a circle, for example, 270 l or at least 250, and there is a field coil I3 for producing magnetic flux across the gap I2 within which the coil II is arranged to travel. Although this arrangement of coils may be used for Various types of dynamometer instruments such as alter- 15 nating and direct current voltmeters and ammeters, for example, it is particularly useful with wattmeters and Ivshall describe the arrangement with particular reference to its use as a wattmeter. For this purpose it is usually most convenient to wind the movable coil I I with relatively fine wire connected in series with a suitable multiplier or resistance coil to serve as the potential coil of the wattmeter and to wind the eld coil with a suitable number of turns of heavy wire to 25 serve as the current coil of the wattmeter.

In order to obtain a wide angle of movement of the rotatable current conducting coil II, the field iron of the instrument is arranged with an inner annular pole piece I4 which is very nearly sur- 30 rounded by an outer pole piece I5. One side of the coil I4 and the shaft I6 to which the said side of the coil II is secured pass through the center of the annular pole piece I 4. The outer pole piece I5,is integral with a field member I1 which, in the arrangement shown, may be generally in the shape of a C with the pole piece I5 constituting a projection extending inwardly from thexnid. portion of the C, the projection or polepiecei'l having a circular notch therein for the inner pole 4 piece Il. 'I'he iield iron is completed bythe core I8 which is generally in the shape of a U with the yoke portion forming the annular inner pole piece Il and with leg portions I9 and "'20 spaced to leave an air gap between them. 45

'I'he end portions 2'I and 22 of the C-shaped ileld I1 approach very closely to the sides of the ends I! and 20 of the U-shaped core I8. Preferably the end portions of the field I1 and the core I8 do not come into immediate contact but leave 50 perceptible air gaps 23 and 24 for the purpose of reducing vfield saturation and overcoming wave form errors of the instrument. In the preferred arrangement which I have illustrated, although the air gaps 23 and 24 are relatively small andv 55 less than-the air gap I2 within which the outer side of the movable coil I I travels, the air gap I2inturnislessinlengththanthoairgap 2l between the leg portions Il and 2l o! the U- shaped core I8.

It will be understood, of course, that the precise shapes o f the field and core members are immaterial and that any shape may be employed which would provide a iiux conducting path from the outer pole piece I5 to the inner pole piece Il. However, in order to avoid having the rotatable coil II subject to having force applied to it solely due to the action of current flowing through it, I prefer to have the core I8 constructed in some form of the nature of that shown where the portion of the material connected to the pole piece I4 is divided so as to have the air gap 25 between said divided portions. and preferably the core I5 has a relatively symmetrical configuration at least in the portion thereof close to the inner annular pole piece Il. It will be seen that in the arrangement illustrated the entire ileld structure is relatively symmetrical, so that flux produced by the field coil I3 mounted on the neck 25 of the pole piece I5 divides substantially equally between the branches 21 and 28 and between the leg portions I5 and 2II to the inner annular pole piece I4, where the two flux paths join, and the ux passes across the gap I2, back to the outer pole piece I5. The annular pole piece Il may be regarded as having a gap 25 in the annulus with the core portions I9 and 20 extending from either side of said gap 25. My invention, however, is not limited to this precise arrange- `ment since the air gap I2 is relatively uniform and causes the flux to distribute itself substantially evenly across the length of the air gap I2.

Although I have referred to the eld members as ileld irons, it will be understood of course that my invention is not limited to the use of iron for the flux conducting portion of the apparatus but that any suitable material having relatively high permeability of 'the lorder of that of soft iron may be employed. I have found, for example, that an alloy of about 47% nickel and the vbalance iron is satisfactory. In connection with alternating current instruments, it will be understood of course that it is advantageous to have the field I1 and the core I8 constructed in the form of stacked laminations.` By making the annular length of the circular opening or notch I2 in the pole piece I5 approximately 270 as shown, it will be seen that even with the coil II of finite width, a movement of the element exceeding 250 may be obtained.

For the purpose of extending the current range and also further decreasing the tendency of the potential coil II to deflect when there is no current in the current coil I3, I preferably modify the shapel of some of the core laminations as illustrated in`Flg. 4, and round oi! decidedly the shape shown by the outline C (Fig. 3).

corners I5 of the field pole piece I5. In an arrangement which I have found to operate successfully, every third lamination of the core I5 has the shape shown by the outline A (Fig. 4) with the shelf portion B, instead of having the The portions B of the laminations of the shape A power of a wattmeter. It is important that a wattmeter having a five-ampere current coil, for example, should be able to measure full scale watts at ten amperes. for example, without an appreciable error in indication.

With the particular design illustrated in the drawing using lonly `core laminations of the shape C, an error of approximately 1.5% may be encountered at ten amperes in the case of an instrument having a five-ampere current coil. 4It is not feasible to attempt to decrease the error by decreasing the current flux and increasing the potential iiux because this would result in tending to increase the voltage errors. Even with the voltage errors made relatively small by the symmetrical design illustrated. there is still a slight tendency for the movable coil II to swing toward the center of the scale so that the reluctances of the portions of the air gap on either side of the coil will be balanced. Obviously this effect would be increased by modifying the design to make the potential flux relatively excessive.

By the use of the shelf portions or projections B in the laminations A, current errors are reduced in the following manner. The projections B, by forming a magnetic shunt across the air gap, reduce the flux in the gap. However, when the current is increased to a value approaching ten amperes (assuming a five-ampere coil) the ilux density becomes so great that the projections become saturated and shunt less ux across the gap. 'Ihis compensates for current errors which would otherwise occur due to increasing saturation of the magnetic circuit as a whole and the decreasing ratio of magnetic iiux to current for the higher values of current. The use of lam-v inations shaped substantially as at A is found to produce flat compensation at ten amperes and a reduction of the current error to less than one per cent up to twelve amperes, in the case of an instrument having a five-ampere coil. The reduction of the reluctance of the air gap at the ends of the scale also decreases the tendency of the coil II to swing to its center scale position. In this manner both the current and voltage errors are appreciably reduced by the modication in the shape of the core laminations.

In Fig. 5 there is illustrated the principle of operation of the arrangement for diminishing current errors. 'I'he particular shape and construction of laminations shown are found to give satisfactory results using the nickel-iron composition of approximately 47% nickel. This alloy has a magnetization curve shaped approximately as shown by the curve E in Fig. 5 with the upper portion F departing materially from a straight line represented by the dotted continuation of the lower portion of the curve. In the case of the five-ampere instrument the point of departure from linearity occurs at approximately the eightampere point. 'I'he curve G represents the portion of the ux shunted by the projections B of the A-shaped laminations, and itwill be observed that in these laminations saturation begins also approximately at the eight-ampere point, which is accomplished by `the suitable selection oi' the number and shape of the laminations. Beyond j the point of beginning saturation the shunted the apparatus will be seen more readily from Fig. 2'showing an enclosing casing 29, partially broken away, in which the E-shaped frame is secured for supporting the parts of the instrument. It will be seen that the frame 39 includes a pair of relativelyA long, narrow, upwardly-extending arms 3I and 32 and, in the ends of these arms, suitable bearings or jewels 33 and 34 are mounted, which have hard surfaced cup-shaped depressions in which mechanical pivot points 35 and 36 formed at the ends of the shaft I6 are permitted to rotate. A relatively wide and spreading arm or bracket 31 is provided at the intermediate portion of the frame 30 for supporting the field laminations I1 by means of suitable bolts 33. The core laminations I8 are likewise secured to the bracket 31 by means of suitable bolts 39.

A pointer 40, and if desired, a suitable damping vane, not shown, are secured to the shaft I6 and it will be understood that suitable vmeans such as lead-in spirals, not shown, for connecting the potential terminals of rthe instrument to the rotatable coil II and a biasing spring, not shown, are also provided. The scale, not shown, for cooperation with the pointer 40, it will be understood, is to be mounted within the front cover 4I of the casing 29 and may be secured to the arm 3 I The construction lends itself to convenient and expeditious assembly. The assembly may be carried out in the following manner. After the coil II has been mounted upon the shaft I6 the shaft I6 may be inserted between the bearings 33 and 34 and secured by a suitable longitudinal adjustment of the bearings. By turning the coil II to a horizontal position, core I8 may be slipped through the coil II and over the shaft I6, whereupon the core I8 is bolted to the bracket 31 of the frame 30. After the eld coil I3 has been slipped on to the pole piece I5 and secured thereto, the field member I1 may be passed longitudinally of the shaft I6 past the arm 32 around the pole piece I4 and is also bolted to the bracket 31. It will be understood that other parts, not shown, such as the scale plate, etc. may be secured to the front bracket 3I of the frame 30 and that additional supporting posts 42 may be provided for securing the scale plate, as well as the back plate 43 of the casing 29 to the field structure for increasing the rigidity of the assembly.

If the apparatus is to be used as a voltmeter or an ammeter, it will be understood that the coils II and I3 may be connected in series, whereas if it is to be used as a wattmeter, the field coil I3 will be connected in series to the load directly or to a suitable transformer and the potential coil II will be connected between the lines of the circuit tested or between the output terminals of the potential transformer in series with a suitable multiplier resistance. Assuming that the apparatus is used as a wattmeter it will be seen that the current flowing through the field coil I3 produces a magnetic fiux crossing the air gap I2 and that the current flowing through the conductors on the outer side of the coil II reacts with this ux to produce a torque causing the shaft I6 and the pointer to deflect in opposition to the biasing spring, not shown. There is a tendency for the current owing through the coil I I to cause a ux to pass through the stationary iron of the apparatus, up one leg I9 of the U-shaped core I8, down leg 20. through the branch 28, and down branch'21 to the end of the core I8, or vice versa. 'I'his ux, however', serves no particular purpose as itis only the flux from the coil II crossing the air gap I2 and reacting with the torque produced by the coil I3 that produces torque. If the idle fiux produced by the coil II, however, should be excessive it would tend to increase the inductance and decrease the power factor of the potential circuit and might also produce saturation effects which, in turn, may lead to wave form errors. To obviate this tendency, the air gaps 23 and 24 are provided between the end portions 2| and 22 of the field member I1 and portions I9 and 20 ofthe core I9.

In accordance with the provisions ofthe patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof but I desire to have it understood that the apparatus shown is only illustrative and. that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A long range dynamometer instrument comprising a field structure including a pair of pole pieces with an annular air gap therebetween and portions joining said pole pieces to form a magnetic circuit completed by passage of flux across said air gap, a field coil linking said field structure, at least one of said pole pieces being substantially annular in shape with a gap therein with branched circuit portions extending from the annular pole piece from either side of the gap therein to the flux conducting portion of the field structure, and a rotatable coil linking said annu- 'lar pole piece with one side passing through it and an axisof rotation passing through said annular pole piece, one of said pole pieces being provided with flux shuntng members at the ends of said air gap, said fiux shuntng members having asmaller cross-section than the main portion of the magnetic circuit and tending to saturate when the rated current of the field coil is exceeded.

2. A long range dynamometer instrument comprising a field structure built up of laminations including a pair of pole pieces with an annular air gap therebetween and portions joining said polel pieces to form a magnetic circuit completed by passage of iiux across said air gap, a eld coil linking said field structure, at least one of said pole pieces being substantially annular in shape, and a rotatable coil linking said annular pole piece with one side passing through it and an axis of rotation passing through said annular pole piece, the laminations of said field structure including laminations having projecting portions at theA ends of saidannular air gap shuntng said air gap and tending to become saturated.

3. A dynamometer instrument comprising a field structure composed of magnetizable material including a pair of pole pieces with an annular air gap therebetween and portions joining said pole pieces to form a magnetic circuit ccmpleted by passage of fiux across said air gap, a field coil linking said field structure, and a movable coil consisting of current conductors so mounted as to travel through said air gap as the coil moves, said field structure including a saturable portion shuntng the air gap.

RALPH M. HOWELL. 

